I have run into an issue where I have multiple asynchronous requests occuring which grab images and information from the Facebook API and my Firebase database. I want to perform all my asynchronous requests, then store all that data that I grabbed from the Facebook API/Firebase database into one entire object which I can quickly load. I have set up completion handlers for every asynchronous request which I thought forces the program to "wait" until the request is complete and then have the program continue, but that doesn't seem to work for me. Below is my attempt:
func setupEvents(completion: (result: Bool, Event: Event) -> Void){
// Get a reference to Events
eventsReference = Firebase(url:"<DB Name>")
eventAttendeesRef = Firebase(url:"<DB Name>")
//Read the data at our posts reference
println("Event References: \(eventsReference)")
eventsReference.observeEventType(FEventType.ChildAdded, withBlock: { (snapshot) -> Void in
let eventName = snapshot.value["eventName"] as? String
let eventLocation = snapshot.value["eventLocation"] as? String
let eventCreator = snapshot.value["eventCreator"] as? String
var attendees: NSMutableDictionary = [:]
var attendeesImages = [UIImage]()
let attendee: NSMutableDictionary = [:]
let group = dispatch_group_create()
//Get attendees first
dispatch_group_enter(group)
self.getAttendees(snapshot.key as String, completion:{ (result, name, objectID) -> Void in
if(result == true){
println("Finished grabbing \(name!) \(objectID!)")
attendees.addEntriesFromDictionary(attendee as [NSObject : AnyObject])
}
else {
println("False")
}
dispatch_group_leave(group)
})
//Get attendees photos
dispatch_group_enter(group)
self.getAttendeesPictures(attendee, completion: { (result, image) -> Void in
if result == true {
println("Finished getting attendee photos. Now to store into Event object.")
attendeesImages.append(image!)
}
else{
println("false")
}
dispatch_group_leave(group)
})
dispatch_group_notify(group, dispatch_get_main_queue()) {
println("both requests done")
//Maintain array snapshot keys
self.eventIDs.append(snapshot.key)
if snapshot != nil {
let event = Event(eventName: eventName, eventLocation:eventLocation, eventPhoto:eventPhoto, fromDate:fromDate, fromTime:fromTime, toDate:toDate, toTime:toTime, attendees: attendees, attendeesImages:attendeesImages, attendeesImagesTest: attendeesImagesTest, privacy:privacy, eventCreator: eventCreator, eventCreatorID: eventCreatorID)
println("Event: \(event)")
completion(result: true, Event: event)
}
}
}) { (error) -> Void in
println(error.description)
}
}
I know I have my completion handlers set correctly as I have tested in my program. However, what I want is that only after both the getAttendees and getAttendeesPictures function completes, I then want to store all the information I grabbed the snapshot, getAttendees, and getAttendeesPictures function and store them into an event object. Any ideas on how to accomplish this? I've tried to look into dispatch_groups to help me handle this via this link: Checking for multiple asynchronous responses from Alamofire and Swift but my program seems to only execute the getAttendees function but not the getAttendeesPictures function. Below are also the getAttendees and getAttendeesPictures functions:
func getAttendees(child: String, completion: (result: Bool, name: String?, objectID: String?) -> Void){
//Get event attendees of particular event
var attendeesReference = self.eventAttendeesRef.childByAppendingPath(child)
println("Loading event attendees")
//Get all event attendees
attendeesReference.observeEventType(FEventType.ChildAdded, withBlock: { (snapshot) -> Void in
let name = snapshot.value.objectForKey("name") as? String
let objectID = snapshot.value.objectForKey("objectID") as? String
println("Name: \(name) Object ID: \(objectID)")
completion(result: true, name: name, objectID: objectID)
}) { (error) -> Void in
println(error.description)
}
func getAttendeesPictures(attendees: NSMutableDictionary, completion: (result: Bool, image: UIImage?)-> Void){
println("Attendees Count: \(attendees.count)")
for (key, value) in attendees{
let url = NSURL(string: "https://graph.facebook.com/\(key)/picture?type=large")
println("URL: \(url)")
let urlRequest = NSURLRequest(URL: url!)
//Asynchronous request to display image
NSURLConnection.sendAsynchronousRequest(urlRequest, queue: NSOperationQueue.mainQueue()) { (response:NSURLResponse!, data:NSData!, error:NSError!) -> Void in
if error != nil{
println("Error: \(error)")
}
// Display the image
let image = UIImage(data: data)
if(image != nil){
completion(result: true, image: image)
}
}
}
}
For users seeking answer to question in title then use of dispatch_group and GCD outlined here: i.e embedding one group inside the notification method of another dispatch_group is valid. Another way to go at a higher level would be NSOperations and dependencies which would also give further control such as canceling operations.
Outline:
func doStuffonObjectsProcessAndComplete(arrayOfObjectsToProcess: Array) -> Void){
let firstGroup = dispatch_group_create()
for object in arrayOfObjectsToProcess {
dispatch_group_enter(firstGroup)
doStuffToObject(object, completion:{ (success) in
if(success){
// doing stuff success
}
else {
// doing stuff fail
}
// regardless, we leave the group letting GCD know we finished this bit of work
dispatch_group_leave(firstGroup)
})
}
// called once all code blocks entered into group have left
dispatch_group_notify(firstGroup, dispatch_get_main_queue()) {
let processGroup = dispatch_group_create()
for object in arrayOfObjectsToProcess {
dispatch_group_enter(processGroup)
processObject(object, completion:{ (success) in
if(success){
// processing stuff success
}
else {
// processing stuff fail
}
// regardless, we leave the group letting GCD know we finished this bit of work
dispatch_group_leave(processGroup)
})
}
dispatch_group_notify(processGroup, dispatch_get_main_queue()) {
print("All Done and Processed, so load data now")
}
}
}
The remainder of this answer is specific to this codebase.
There seem to be a few problems here:
The getAttendees function takes an event child and returns an objectID and Name which are both Strings? Shouldn't this method return an array of attendees? If not, then what is the objectID that is returned?
Once an array of attendees is returned, then you can process them in a group to get the pictures.
The getAttendeesPictures eventually returns UIImages from Facebook. It's probably best to cache these out to the disk and pass path ref - keeping all these fetched images around is bad for memory, and depending on size and number, may quickly lead to problems.
Some examples:
func getAttendees(child: String, completion: (result: Bool, attendees: Array?) -> Void){
let newArrayOfAttendees = []()
// Get event attendees of particular event
// process attendees and package into an Array (or Dictionary)
// completion
completion(true, attendees: newArrayOfAttendees)
}
func getAttendeesPictures(attendees: Array, completion: (result: Bool, attendees: Array)-> Void){
println("Attendees Count: \(attendees.count)")
let picturesGroup = dispatch_group_create()
for attendee in attendees{
// for each attendee enter group
dispatch_group_enter(picturesGroup)
let key = attendee.objectID
let url = NSURL(string: "https://graph.facebook.com/\(key)/picture?type=large")
let urlRequest = NSURLRequest(URL: url!)
//Asynchronous request to display image
NSURLConnection.sendAsynchronousRequest(urlRequest, queue: NSOperationQueue.mainQueue()) { (response:NSURLResponse!, data:NSData!, error:NSError!) -> Void in
if error != nil{
println("Error: \(error)")
}
// Display the image
let image = UIImage(data: data)
if(image != nil){
attendee.image = image
}
dispatch_group_leave(picturesGroup)
}
}
dispatch_group_notify(picturesGroup, dispatch_get_main_queue()) {
completion(true, attendees: attendees)
}
}
func setupEvents(completion: (result: Bool, Event: Event) -> Void){
// get event info and then for each event...
getAttendees(child:snapshot.key, completion: { (result, attendeesReturned) in
if result {
self.getAttendeesPictures(attendees: attendeesReturned, completion: { (result, attendees) in
// do something with completed array and attendees
}
}
else {
}
})
}
The above code is just an outline, but hopefully points you in the right direction.
The two requests are executing at the same time, so there is no attendees to get pictures from when the second request executes, if the getAttendees completion closure is going to be called multiple times then you can do something like this:
let group = dispatch_group_create()
for key in keys {
dispatch_group_enter(group)
self.getAttendee(key as String, completion:{ (result, attendee) in
if(result == true){
attendees.addEntriesFromDictionary(attendee)
self.getAttendeesPictures(attendee, completion: { (result, image) in
if result == true {
attendeesImages.append(image!)
}
dispatch_group_leave(group)
})
} else {
dispatch_group_leave(group)
}
})
}
dispatch_group_notify(group, dispatch_get_main_queue()) {}
If the result of the first request is the complete set of attendees you don't even need to use GCD, just call getAttendeesPictures inside the completion closure.
This code doesn't exactly uses the same variables and methods of the original code, it only gives the idea.
Hope it helps!
While there is definitely solution with using GCD and stuff around it, synchronization in general is pain and the more your code gets complicated, the more problems it will start showing - but I think there is one-for-all solution to that: Bolts framework from Facebook (both for android na iOS)
Bolts Framework usage
So what is so magical about it? Well, it lets you create "Tasks", and then chain them. The method in particular that you are interested in is taskForCompletionOfAllTasks: , which is made for parallel processing, just what you need. I wrote a little example for you which you can adjust to your needs:
func fetchAllInformation() -> BFTask {
// First, create all tasks (if you need more, than just create more, it is as easy as that
var task1 = BFTaskCompletionSource()
var task2 = BFTaskCompletionSource()
var tasks = [task1, task2]
// What you do, is you set result / error to tasks and the propagate in the chain upwards (it is either result, or error)
// You run task 1 in background
API.instance.fetchFirstDetailsInBackgroundWithBlock {
(object: AnyObject!, error: NSError!) -> Void in
// On error or on success, you assign result to task (whatever you want)
if error == nil {
task1.setResult(object)
} else {
task1.setError(error)
}
}
// You run task 2 in background
API.instance.fetchSecondDetailsInBackgroundWithBlock {
(object: AnyObject!, error: NSError!) -> Void in
// On error or on success, you assign result to task (whatever you want)
if error == nil {
task2.setResult(object)
} else {
task2.setError(error)
}
}
// Now you return new task, which will continue ONLY if all the tasks ended
return BFTask(forCompletionOfAllTasks: tasks)
}
Once you have main method done, you can use bolts chaining magic:
func processFullObject() {
// Once you have main method done, you can use bolts chaining magic
self.fetchAllInformation().continueWithBlock { (task : BFTask!) -> AnyObject! in
// All the information fetched, do something with result and probably with information along the way
self.updateObject()
}
}
The Bolts framework documentation / README covers basically everything there is to know about it and it is quite extensive, so I would suggest you to go through it - it is very easy to use once you get the basics. I personally use it for exactly this, and it is a blast. This answer will hopefully provide you with different solution and approach, possibly a cleaner one.
There is something wrong with this conceptually. It sounds like you want to wait until both of these functions complete before doing something else, but what you haven't explained is that getAttendeesPictures depends on the outcome of getAttendees. That means what you really want to do it execute one asynchronous block, then execute a second asynchronous block with the output of the first, and then execute your final completion block when both are finished.
GCD is not particularly suited for this; you're better of using NSOperationQueue with NSBlockOperations. There are two distinct advantages to this over GCD:
NSOperation uses familiar object-oriented syntax compared to GCD's c-type functions, so it's pretty easy to write and understand.
Operations in the queue can have explicit dependencies on one another, so you can make it clear that e.g. operation B will only be executed after operation A is complete.
There is a great writeup of this by NSHipster which I'd recommend you go read. It's talked about mostly in the abstract, but what you want to do is use NSBlockOperation to create two block operations, one for executing getAttendees and one for executing getAttendeesPictures, and then make it explicit that the second block depends on the first before adding them both to a queue. They will then both execute and you can use a completion block on the second operation to do something once both have completed.
Dave Roberts is right in his response though: an immediate problem with the code is that you don't use the output of the getAttendees function to actually create any attendees. Perhaps this part of the code is missing, but from what I can see the name and objectID are just printed out. If you want to pass something useful into the getAttendeesPictures function you will need to fix this part first.
This is off the top of my head. The idea is to read and handle new asyc data only when all of the nested blocks complete.
We leverage a while loop to handle waiting for a signal to read the next set of data.
The outside while loop continues as long as done equals false. And nothing is really going on, other than consuming cpu cycles while it waits. The if inside the loop will only be trigged (set to true) when all of the attendees have been read.
Meanwhile inside the loop we work through nested blocks, reading in the attendee and then when that completes, read their picture, and when that completes read the firebase data. Finally once we have all data from the prior blocks we stuff the data into an object which is then added to the dictionary. At that time it is determined if we are finished reading attendees and if so, bail completely. If not, we read the next attendee.
(this is conceptual)
done = false
readyToReadNextAttendee = true
while ( done == false )
{
if (readyToReadNextAttendee == true ) {
readyToReadNextAttendee = false
readAttendee
readPicture
readFirebase {
putDataIntoObject
addObjectToDictionary
if finishedReadingAttendees {
done = true
} else {
readyToReadNextAttendee = true
}
}
}
}
If you have the option of reading in all of the attendees first, you could iterate over and array as well, not reading the next index until readyToReadNextAttendee = true
One Idea i have used is to place an if statement check inside the query statement call back and place the query statement call back in a for loop (so you can loop through all of your queries), so the if statement should check if this the last call back expected, then you should execute a return statement or a deferred.resolve statement, the following is a concept code.
var list=fooKeys //list of keys (requests) i want to fetch form firebase
var array=[] // This is the array that will hold the result of all requests
for(i=xyz;loop breaking condition; i++){
Ref = new Firebase("https://yourlink.firebaseio.com/foo/" + fooKeys[i]);
Ref.once("value", function (data) {
array.push(data.val());
if(loop breaking condition == true){
//This mean that we looped over all items
return array; //or deferred.resolve(array);
}
})
}
Putting this code in a function and call it asynchronously will give you the ability to wait for the whole results before proceed in doing other stuff.
Hope you (and the others) find this beneficial.
Related
I have two independent observables. I need to perform some operation when both of them are complete and each of them provided an array.
let myObj1Array = myObj1Manager.getMyObj1List()//returns Observable<[MyObj1]>
let myObj2Array = myObj2Manager.getMyObj2List()//returns Observable<[MyObj2]>
Now I need to compare values of myObj1Array and myObj2Array and on the basis of that create another array using values from both arrays. I know how to subscribe 1 variable but not sure how to observe completion of two different arrays.
Edit:
I also tried following but I get values only from first array:
let myObj1Array = myObj1Manager.getMyObj1List()
let myObj2Array = myObj1Array.flatMap { _ in myObj2Manager.getMyObj2List() }
Observable.combineLatest(myObj1Array, myObj2Array)
.subscribe(onNext: { (sss, sds) in
print(sss)
})
.addDisposableTo(disposeBag)
I am actually kind of clueless about how to handle such scenario.
Edit2:
function to get the observables in first array:
func getMyObj1List() -> Observable<[MyObj1]> {
return Observable.create { observer -> Disposable in
self.specialsRest.getMyObj1List { response, error in
if let error = error {
observer.onError(Exception(error))
return
}
guard let saleItems = MyObj1.decode(data: response?.data) else {
observer.onError(Exception("Could not decode specials!"))
return
}
queueBackground.async {
observer.onNext(saleItems)
observer.onCompleted()
}
}
return Disposables.create { self.specialsRest.cancel() }
}
}
DispatchGroup is probably the way to go here.
https://developer.apple.com/documentation/dispatch/dispatchgroup
When all work items finish executing, the group executes its completion handler. You can also wait synchronously for all tasks in the group to finish executing.
var dg:DispatchGroup = DispatchGroup()
//Wherever you start your observables.
//Start Observer1
dg.enter()
//Start Observer2
dg.enter()
...
...
...
//Wherever you retrieve data
SomeAsyncFuncForObserver1 {
//Get Data
dg.leave()
}
SomeAsyncFuncForObserver2 {
//Get Data
dg.leave()
}
dg.notify(queue: .main) {
print("all finished.")
}
I believe you need to use zip instead of combineLatest. From the docs
The CombineLatest operator behaves in a similar way to Zip, but while
Zip emits items only when each of the zipped source Observables have
emitted a previously unzipped item, CombineLatest emits an item
whenever any of the source Observables emits an item (so long as each
of the source Observables has emitted at least one item).
Observable
.zip(myObj1Array, myObj2Array)
.subscribe(onNext: { (sss, sds) in
print(sss)
})
.addDisposableTo(disposeBag)
I'm rather new at swift and have been doing some research on how to answer this question myself since I want to learn, but I am completely stumped.
I have a function which requests data from a server, and after the data is received, a completion handler is executed which parses the data. Within the previously mentioned completion handler, another function is called which is passed a completion handler itself.
For some reason, the function call within the function is being being skipped, and being finished after the first completion handler is fully executed. This might make more sense with the code below:
func loadSites(forceDownload: Bool){
self.inspectionSites = MyData.getLocallyStoredInspectionSites()
if self.inspectionSites.count < 1 || forceDownload {
self.http.requestSites({(sitesAcquired, jsonObject) -> Void in
guard sitesAcquired else{
SwiftOverlays.removeAllBlockingOverlays()
MyAlertController.alert("Unable to acquire sites from server or locally")
return
}
let result = jsonObject
for (_,subJson):(String, JSON) in result!.dictionaryValue {
let site = InspectionSite()
site.name = subJson[self.currentIndex]["name"].string!
site.city = subJson[self.currentIndex]["city"].string!
site.address = subJson[self.currentIndex]["address"].string!
site.state = subJson[self.currentIndex]["state"].string!
site.zip = subJson[self.currentIndex]["zip"].stringValue
site.siteId = subJson[self.currentIndex]["id"].string!
objc_sync_enter(self) //SAW A STACKOVERFLOW POST WITH THIS, THOUGHT IT MIGHT HELP
MyLocation.geoCodeSite(site, callback:{(coordinates) -> Void in
print("YO!!!! GEOCODING SITE!")
self.localLat = coordinates["lat"]!
self.localLon = coordinates["lon"]!
})
objc_sync_exit(self)
for type in subJson[self.currentIndex]["inspection_types"]{
let newType = InspectionType()
newType.name = type.1["name"].string!
newType.id = type.1["id"].string!
site.inspectionTypes.append(newType)
}
site.lat = self.localLat
print("HEYY!!!! ASSIGNING COORDS")
site.lon = self.localLon
let address = "\(site.address), \(site.city), \(site.state) \(site.zip)"
site.title = site.name
site.subtitle = address
MyData.persistInspectionSite(site)
self.currentIndex++
}
self.inspectionSites = MyData.getLocallyStoredInspectionSites()
SwiftOverlays.removeAllBlockingOverlays()
self.showSitesOnMap(self.proteanMap)
})
}else{
SwiftOverlays.removeAllBlockingOverlays()
self.showSitesOnMap(self.proteanMap)
}
}
I added those print statements which print "YOOO" and "HEYYY" just so I could see what was being executed first, and "HEYY" is always first. I just need to make sure that the geocoding always happens before the object is persisted. I saw a stackoverflow post which mentioned objc_sync_enter(self) for synchronous operation, but im not even sure if it's what I need.
This is the function which geocodes the site (incase it helps):
class func geoCodeSite(site: InspectionSite, callback: ((coordinates: Dictionary<String, String>)->Void)?) {
let geocoder = CLGeocoder()
let address: String = "\(site.address), \(site.city), \(site.state) \(site.zip)"
print(address)
geocoder.geocodeAddressString(address, completionHandler: {(placemarks, error) -> Void in
if((error) != nil){
print("Error", error)
}
if let placemark = placemarks?.first {
MyLocation.mLat = String(stringInterpolationSegment:placemark.location!.coordinate.latitude)
MyLocation.mLon = String(stringInterpolationSegment:placemark.location!.coordinate.longitude)
MyLocation.coordinates = ["lat":mLat, "lon":mLon]
print(MyLocation.coordinates)
callback?(coordinates: MyLocation.coordinates)
}
})
}
I think the behaviour your seeing is expected. You have two levels of asynchronous methods:
requestSites
geoCodeSite
Since the geoCodeSite method is also asynchronous, its callback is executed well after the line:
MyData.persistInspectionSite(site)
So your problem is how to wait till all InspectionSites have geocoded before persisting the site, right?
Dispatch groups can be used to detect when multiple asynchronous events have finished, see my answer here.
How to Implement Dispatch Groups
dispatch_groups are used to fire a callback when multiple async callbacks have finished. In your case, you need to wait for all geoCodeSite async callbacks to complete before persisting your site.
So, create a dispatch group, firing off your geoCodeSite calls, and implement the dispatch callback inside of which you can persist your geocoded sites.
var myGroup = dispatch_group_create()
dispatch_group_enter(myGroup)
...
fire off your geoCodeSite async callbacks
...
dispatch_group_notify(myGroup, dispatch_get_main_queue(), {
// all sites are now geocoded, we can now persist site
})
Don't forget to add
dispatch_group_leave(myGroup)
inside the closure of geoCodeSite! Otherwise dispatch_group will never know when your async call finish.
I'm trying to generate an array of PFObjects called 'areaList'. I've been researching this quite a bit and understand that I could benefit from using a completion handler to handle the asynchronous nature of the loaded results. My ask, specifically, is to get some guidance on what I'm doing wrong as well as potential tips on how to achieve the result "better".
Here is my query function with completion handler:
func loadAreasNew(completion: (result: Bool) -> ()) -> [Area] {
var areaList = self.areaList
let areaQuery = PFQuery(className: "Area")
areaQuery.findObjectsInBackgroundWithBlock {
(areas: [PFObject]?, error: NSError?) -> Void in
if error == nil {
for area in areas! {
let areaToAdd = area as! Area
areaList.append(areaToAdd)
// print(areaList) // this prints the list each time
// print(areaToAdd) // this prints the converted Area in the iteration
// print(area) // this prints the PFObject in the iteration
if areaList.count == areas!.count {
completion(result: true)
} else {
completion(result: false)
}
}
} else {
print("There was an error")
}
}
return areaList
}
Here is how I'm attempting to call it in viewDidLoad:
loadAreasNew { (result) -> () in
if (result == true) {
print(self.areaList)
} else {
print("Didn't Work")
}
}
I assigned this variable before viewDidLoad:
var areaList = [Area]()
In the console, I get the following:
Didn't Work
Didn't Work
Didn't Work
Didn't Work
[]
Representing the 5 items that I know are there in Parse...
This is an interesting question. First off, PFQuery basically has a built in completion handler, which is quiet nice! As you probably know, all of the code within the areaQuery.findObjectsInBackgroundWithBlock {...} triggers AFTER the server response. A completion most often serves the purpose of creating a block, with the ability of asynchronously returning data and errors.
Best practice would (IMO) to just call the code that you want to use with the results from your PFQuery right after your area appending loop (which I'm gonna take out because I'm picky like that), like so:
func loadAreasNew() {
var areaList = self.areaList
let areaQuery = PFQuery(className: "Area")
areaQuery.findObjectsInBackgroundWithBlock {
(areas: [PFObject]?, error: NSError?) -> Void in
if error == nil {
let areasFormatted = areas! As [Areas]
areasList += areasFormatted
//Something like this
self.codeINeedAreasFor(areasList)
}
} else {
print(error)
}
}
}
HOWEVER! If you really feel the need to use some completion handlers, check out this other answer for more info on how to use them. But keep in mind all tools have a time and a place...
There are a few issues here.
Your completion handler doesn't require you to define the name for your completion handler's parameters, so you could easily use completion: (Bool) -> ()
Further in your function, you're returning areaList. This should be put through the completion handler like this onComplete(areaList) and change your completion handler parameter to expect your area list.
Then, when you call your function, it could look more like this :
loadAreasNew { result in
if (result == true) {
print(self.areaList)
} else {
print("Didn't Work")
}
}
Here is my concern:
1) Don't pass in a local variable and make the function return it, it's meaningless and danger.
You may want to initiate an empty array and make your fetch, then "return" it.
2) The fetch request is processed in background, you will have no idea when it will have finished. If you return the array immediately it will always be an empty array.
Put the "return" in your completion too.
3) Parse already has a distance checking method, you don't have to do it manually. aPARSEQUERRY.where(key:,nearGeoPoint:,inKilometers:)
I will rewrite the function as:
func loadNewAreas(completion:([Area],err?)->()){
let areaQuery = PFQuery(className: "Area")
areaQuery.where("location",nearGeoPoint:MYCURRENTLOCATION,inKilometers:50)
areaQuery.findObjectInBackgroundWithBlock(){objects,err
if objects.count == 0
{
completion([],err)
}
let areas = Area.areasFromPFObjects(objects)
completion(areas,err)
}
}
Please help me to solve this problem - after a lot of (not so efficent...) search I can't do this alone.
I have the following methods:
showLoadingAnimation()
to show the loading animation while background tasks are running
hideLoadingAnimation()
to hide the loading animation as soon as all the background tasks are finished
getUserFacebookData()
to get Facebook-user data
uploadUserFacebookDataToServer()
to upload the Facebook-user data to the server (and perform tasks with them).
What I want to perform:
Show up the loading animation: showLoadingAnimation()
Get the user data from Facebook: getFacebookData()
Wait until these data are being downloaded
As soon as the Facebook-user data are being download, upload these data to the server: uploadUserFacebookDataToServer()
Wait untile these data are being uploaded
Hide the loading animation: hideLoadingAnimation()
Now my problem is, that I don't know how to solve this problem. I know, that I should use sync and/or async tasks, GCD... But I don't know how, and I can't find a proper guide to it.
Could someone explain it to me through these functions?
Thanks!
UPDATE:
Thank you, Zhi-Wei Cai, that was the kind of answer what I was hoping for.
Now it seems to work, the calling order is OK,
but now the problem is the same as the beginning:
uploadUserFacebookDataToServer()
doesn't wait until
getUserFacebookData
downloads the user data from Facebook, that's why it won't be able to work with the necessary data given back from
getUserFacebookData
Any idea? Is there anything to do with dispatch?
UPDATE 2:
As you requested, here are the fuctions. I hope, with these information you can help me to solve this problem and to understand this whole process.
func getFacebookUserData(completionHandler: () -> Void)
{
println("getFacebookUserData")
let graphRequest : FBSDKGraphRequest = FBSDKGraphRequest(graphPath: "me", parameters: nil)
graphRequest.startWithCompletionHandler({ (connection, result, error) -> Void in
if ((error) != nil)
{
// Process error
println("Error: \(error)")
}
else
{
let userID : NSString = result.valueForKey("id") as NSString!
var defaults: NSUserDefaults = NSUserDefaults.standardUserDefaults()
defaults.setObject(userID, forKey: "settings_facebookID")
self.facebookID_fromSettings = userID
}
})
and
func getObjectIDfromFacebookID(completionHandler: () -> Void)
{
println("getObjectIDfromFacebookID")
var query = PFQuery(className:"users")
query.whereKey("facebookID", equalTo:facebookID_fromSettings)
println("getObjectIDfromFacebookID: facebookID: " + facebookID_fromSettings)
query.findObjectsInBackgroundWithBlock {
(objects: [AnyObject]?, error: NSError?) -> Void in
if error == nil {
// The find succeeded.
println("Successfully retrieved \(objects!.count) scores.")
// Do something with the found objects
if (objects!.count == 0) {
// New user, registering
println("New user, registering")
}
else
{
//User is already regsitered, reading out objectID
println("User is already regsitered, reading out objectID")
}
if let objects = objects as? [PFObject] {
for object in objects {
println("objectID: " + object.objectId)
var objectID: String = object.objectId
println(objectID)
var defaults: NSUserDefaults = NSUserDefaults.standardUserDefaults()
defaults.setObject(objectID, forKey: "settings_objectID")
}
}
}
}
completionHandler()
}
So the first function gets the facebookID from the FB-server, but this process takes time, it won't give a result immediately. The second function should work with this data, that's why it should "wait" until the first gives back the requested data.
I can solve this problem by building these 2 fuctions together in one, but that's "not elegant", and I also would like to use this (sync/async dispatch) method in other parts of the project,
Thanks for trying to help me!
You can use completion handlers:
func showLoadingAnimation() {
self.getUserFacebookData({ () -> Void in
self.uploadUserFacebookDataToServer({ () -> Void in
self.hideLoadingAnimation()
})
})
}
func getUserFacebookData(completionHandler: () -> Void) {
println("getUserFacebookData")
completionHandler()
}
func uploadUserFacebookDataToServer(completionHandler: () -> Void) {
println("uploadUserFacebookDataToServer")
completionHandler()
}
func hideLoadingAnimation() {
println("hideLoadingAnimation")
}
Once showLoadingAnimation() is called, the rest will be done asynchronously.
Reference: https://developer.apple.com/library/ios/featuredarticles/Short_Practical_Guide_Blocks/
[EDIT] Use dispatch_group_t
check following code
Step 1: showLoadingAnimation()
Step 2: dispatch_group_t faceBookService = dispatch_group_create();
Step 3:dispatch_group_async_enter(faceBookService,queue,^{
getUserFacebookData()
// The below given line should be inside completion handler or after the above task has finished
dispatch_group_leave(faceBookService);
});
Step 4:dispatch_group_async_enter(faceBookService,queue,^{
uploadUserFacebookDataToServer()
// The below given line should be inside completion handler or after the above task has finished
dispatch_group_leave(faceBookService);
});
Step 5:dispatch_group_notify(faceBookService,dispatch_get_main_queue(),^{
//Called after finishing both tasks.
hideLoadingAnimation()
});
I am writing an application that depends on data from various sites/service, and involves performing calculations based on data from these different sources to produce an end product.
I have written an example class with two functions below that gathers data from the two sources. I have chosen to make the functions different, because sometimes we apply different authentication methods depending on the source, but in this example I have just stripped them down to their simplest form. Both of the functions use Alamofire to fire off and handle the requests.
I then have an initialisation function, which says if we have successfully gathered data from both sources, then load another nib file, otherwise wait up to for seconds, if no response has been returned, then load a server error nib file.
I've tried to make this example as simple as possible. Essentially. This is the kind of logic I would like to follow. Unfortunately it appears this does not currently work in its current implementation.
import Foundation
class GrabData{
var data_source_1:String?
var data_source_2:String?
init(){
// get data from source 1
get_data_1{ data_source_1 in
println("\(data_source_1)")
}
// get data from source 2
get_data_2{ data_source_1 in
println("\(data_source_1)")
}
var timer = 0;
while(timer<5){
if((data_source_1 == nil) && (data_source_2 == nil)){
// do nothing unless 4 seconds has elapsed
if (timer == 4){
// load server error nib
}
}else{
// load another nib, and start manipulating data
}
// sleep for 1 second
sleep(1)
timer = timer+1
}
}
func get_data_1(completionHandler: (String) -> ()) -> () {
if let datasource1 = self.data_source_1{
completionHandler(datasource1)
}else{
var url = "http://somewebsite.com"
Manager.sharedInstance.request(.GET, url).responseString {
(request, response, returnedstring, error) in
println("getting data from source 1")
let datasource1 = returnedstring
self.data_source_1 = datasource1
completionHandler(datasource1!)
}
}
}
func get_data_2(completionHandler: (String) -> ()) -> () {
if let datasource2 = self.data_source_2{
completionHandler(datasource2)
}else{
var url = "http://anotherwebsite.com"
Manager.sharedInstance.request(.GET, url).responseString {
(request, response, returnedstring, error) in
println("getting data from source 2")
let datasource2 = returnedstring
self.data_source_2 = datasource2
completionHandler(datasource2!)
}
}
}
}
I know that i could put the second closure within the first inside the init function, however, I don't think this would be best practice and I am actually pulling from more than 2 sources, so the closure would be n closures deep.
Any help to figuring out the best way to checking if multiple data sources gave a valid response, and handling that appropriately would be much appreciated.
Better than that looping process, which would block the thread, you could use dispatch group to keep track of when the requests were done. So "enter" the group before issuing each of the requests, "leave" the group when the request is done, and set up a "notify" block/closure that will be called when all of the group's tasks are done.
For example, in Swift 3:
let group = DispatchGroup()
group.enter()
retrieveDataFromURL(url1, parameters: firstParameters) {
group.leave()
}
group.enter()
retrieveDataFromURL(url2, parameters: secondParameters) {
group.leave()
}
group.notify(queue: .main) {
print("both requests done")
}
Or, in Swift 2:
let group = dispatch_group_create()
dispatch_group_enter(group)
retrieveDataFromURL(url1, parameters: firstParameters) {
dispatch_group_leave(group)
}
dispatch_group_enter(group)
retrieveDataFromURL(url2, parameters: secondParameters) {
dispatch_group_leave(group)
}
dispatch_group_notify(group, dispatch_get_main_queue()) {
print("both requests done")
}
The other approach is to wrap these requests within an asynchronous NSOperation subclass (making them cancelable, giving you control over constraining the degree of concurrency, etc.), but that's more complicated, so you might want to start with dispatch groups as shown above.